How to monitor render process in a thread-safe manner?
You can add handlers for render complete process: http://www.blender.org/documentation/blender_python_api_2_67b_release/bpy.app.handlers.html#bpy.app.handlers.render_complete

Use that to flip a flag that you can check in your main code that can check when the current render is ready before starting a new one.

Check also render_post (and render_pre) for usage in your add-on.